Color measurement systems for web printing processes are generally known. For example, Graphics Microsystems, Inc., assignee herein, has for many years made and sold AUTOSMART densitometers for color measurement systems and MICROCOLOR ink control systems. More recently, Graphics Microsystems has used color measurement in a closed-loop color control system sold as the AUTOCOLOR color control system.
Until now, the color measurement systems used by Graphics Microsystems have required an operator to place the printed sheet or signature on a measurement system after it is removed from the printing press. It is well known that the preferred method, particularly in web press printing, is to measure color automatically on the press both to save operator time and to provide more prompt and frequent measurements for the color control system.
A number of on-press color measurement systems have been designed, including those made by MacBeth, Finnish Research Center (now the Honeywell system), IGT, and others. All have had only limited use due in large part to an inability to accurately locate a measurement patch on a moving web or sheet or because the measurement patch required was too large to be acceptable, particularly in web printing. Thus, for successful on-press, closed-loop color control, a small measurement patch is necessary, e.g. 2 mm in the running direction and 4 mm in the cross-web direction. A successful system must find the small measurement patch accurately for measurement, and then confirm that the location was accurate enough to use the measurement thus obtained.
Verification of measurements is critical for a successful color control system. Color control with measurements that are incorrect because the measurement was not made in the correct location can increase rather than reduce scrap compared to manual color control. Therefore, a successful color control system must be able to confirm the accuracy of the measurement location before using it for color control.